Many different systems now use thyristors or other switching components connected in an inverter, to receive a d-c input voltage over a d-c bus and provide an output a-c voltage for energizing an electric induction motor. The motor in turn is mechanically connected to drive a load. A d-c bridge or other input component is generally connected to receive a-c voltage over an input line and, as a function of a control signal, to produce an adjustable level of d-c bus voltage for supplying the inverter. The inverter frequency is controlled through a logic system, in which a voltage controlled oscillator or a similar component has its frequency regulated to in turn regulate the frequency of inverter operation, thus controlling the frequency of the inverter voltage.
In such systems it is a general practice to control the inverter voltage amplitude and the inverter voltage frequency in a linear relationship, according to the same control characteristic. This is frequently termed a "constant V/f" operation, or constant volts/hertz operation. This is done to insure that at the higher power levels, there is sufficient current flow in the magnetic motor circuits to maintain the desired torque level. Certain workers in this technology have recognized the possibility of controlling the voltage amplitude and frequency of the inverter output in a manner other than a constant V/f mode. For example, U.S. Pat. No. 3,467,904, entitled "Speed Control System Utilizing Constant-Amplitude Voltage of Variable Frequency to Energize an Electric Motor", which issued Sept. 16, 1969 to the assignee of this invention, teaches the production of an inverter output voltage in which the frequency is varied to control the speed of a motor, while the voltage amplitude is maintained constant. Of course this simplifies the construction of the rectifier input bridge, which can be constructed with passive components to produce a constant d-c voltage on the bus to energize the inverter. However such a system does not afford the flexibility of producing a desired voltage control characteristic which varies in a manner different than the frequency characteristic of the inverter output voltage.
It is therefore a primary object of the present invention to produce an inverter-motor control system for regulating both the frequency and the amplitude of the inverter output voltage, but along different characteristic curves, to achieve a desired overall control function.
It is a more specific object of the invention to provide such an arrangement which, in conjunction with d-c input bridge, regulates the entire system operation in such a manner that the power factor of the system is not debilitated as the system is run down from the 100% load condition to a reduced operating level substantially below 100% load.